Three alarm clock

To find the time when the clocks will next ring together, we have to find LCM of 4, 12 and 20 minutes.

4 = 2²

12 = 2² × 3

20 = 2² × 5

⇒ LCM of 4, 12 and 20 = 2² × 3 × 5 = 60 minutes.

So, the clocks will ring together again after 60 minutes or one hour.